The engine installed in a vehicle is generally coupled to an exhaust gas purification system, which purifies exhaust gas discharged from the engine. The exhaust gas purification system includes a first housing for accommodating a filter and a second housing for accommodating a selective reduction catalyst, for example, as disclosed in Patent Document 1. The first housing is located upstream of the second housing in the flowing direction of exhaust gas. The filter, which is accommodated in the first housing, catches particulate matter contained in exhaust gas. The second housing is located downstream of the first housing in the flowing direction of the exhaust gas. The selective reduction catalyst, which is accommodated in the second housing, reduces NOx contained in the exhaust gas with a reducing agent. FIG. 7 is a schematic diagram showing the structure of a connection between the first housing and an injection device, which injects a reducing agent to the selective reduction catalyst.
As shown in FIG. 7, a first housing 111 includes an inner pipe 112 inside the first housing 111. The basal end of the inner pipe 112 is coupled to the inner circumferential surface of the first housing 111. The distal end of the inner pipe 112 is coupled to a portion around an outlet formed in the first housing 111. The circumferential wall of the inner pipe 112 includes a passage into which the exhaust gas E flows after exiting from the filter. The exhaust gas E that has exited from the filter exits to the outside of the first housing 111 through the interior of the inner pipe 112. An injection device 100 injects a reducing agent R to the exhaust gas E flowing inside the inner pipe 112.
The injection device 100 includes a main body 101 having an injection port 101a, through which the reducing agent R is injected, and a leg portion 102, which extends radially outward from the outer circumferential surface of the main body 101. Mounting holes 102a are formed in the leg portion 102 of the injection device 100, and stud bolts 103 pass through the mounting holes 102a. Internal threads 113a that mesh with the stud bolts 103 are formed in a mounting flange 113, which is located on the outer circumferential surface of the first housing 111. The stud bolts 103, which mesh with the internal threads 113a, fix the leg portion 102 of the injection device 100 to the mounting flange 113. Annular nuts 104 are located in the portions of the stud bolts 103 that protrude from the leg portion 102 away from the injection port 101a. The nuts 104 mesh with the stud bolts 103. The nuts 104 with the stud bolts 103 fix the leg portion 102 of the injection device 100 to the mounting flange 113.